Softball and baseball players often choose different bat weights depending on game rules, style of play, field conditions, environmental conditions, and personal fatigue conditions. For example, players in a league that allows home runs may choose to use a heavier bat, while a player facing more challenging pitchers, or experiencing fatigue late in a game or season, may choose to use a lighter bat.
A bat's “swing weight” can be indicated by its moment of inertia (“MOI”). MOI is the product of: (a) mass, and (b) the square of the distance between the center of the mass and the point from which the mass is pivoted. Mathematically, this is expressed as follows:MOI=ΣMass×(Distance)2 
The MOI dictates that it becomes increasingly difficult to swing a bat as the bat's mass increases or as the center of the bat's mass moves farther from the pivot point of the swing (i.e., farther from the batter's hands). A bat with a lower MOI is easier to swing, resulting in more control or a faster swing that can help the player meet a challenging pitch, while a heavier bat may limit the ability of the player to reach the pitched ball. But for a player with more strength and skill, or less fatigue, an increased MOI allows the player to impart more power on the ball, resulting in longer or faster hits than a bat with a lower MOI. For example, a player may be fresh and strong early in a season, tournament, or game and able to use a bat with a higher MOI, while the player may later need a bat with a lower MOI if the player is experiencing fatigue or is facing a challenging pitcher or adverse environmental conditions.
As a result of changing conditions, rules, or preferences, players may need to carry and use multiple bats to meet their needs. This can add cost and decrease a batter's confidence as a result of variations between different bats.